VanTag: RFID Communication Enhancement using Van Atta Array

RFID has become an attractive platform for battery-free sensing and human-centered systems. However, its practical deployment is often constrained by limited communication range or orientation dependency in real-world conditions, due to weak, omnidirectional backscatter communication. In this paper, we present VanTag, a novel UHF RFID tag architecture that integrates a Van Atta-style retrodirective array with a commodity RFID chip to enhance backscatter communication and improve measurement robustness for sensing. VanTag develops a novel coupler-based differential feed network co-designed with a symmetry-preserving matching circuit to overcome key challenges, including differential-port compatibility, differential impedance matching, and physical size constraints. We prototype VanTag designs and evaluate them on a commercial UHF RFID platform. Measurements show a 4.2 dB increase in radar cross-section, a 40.9% extension in read range, and good robustness to angle and orientation variations, all while preserving full compatibility with commodity readers and standard RFID protocols. These results demonstrate that modulated retrodirective backscatter can provide both communication enhancement and more stable backscatter measurements, offering a practical building block for robust battery-free sensing and IoT systems.